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PT4687N Datasheet(PDF) 11 Page - Texas Instruments |
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PT4687N Datasheet(HTML) 11 Page - Texas Instruments |
11 / 13 page For technical support and more information, see inside back cover or visit www.ti.com Application Notes 0 5 10 15 20 25 30 35 t (milliseconds) Vo1 (2V/Div) Vo2 (2V/Div) IIN (0.5A/Div) Figure 3; Vo1, Vo2 Power-Up Sequence PT4660 & PT4680 Series On/Off Output Voltage Sequencing The output voltages from the PT4660 series of DC/ DC converters are independantly regulated, and are internally sequenced to meet the power-up requirements of popular microprocessor and DSP chipsets. Figure 3 shows the waveforms from a PT4661 after the converter is enabled at t=0s. During power-up, the Vo1 and Vo2 voltage waveforms typically track within 0.4V prior to Vo2 reaching regulation. The waveforms were measured with a 5-Adc resistive load at each output, and with a 48- VDC input source applied. The converter typically produces a fully regulated output within 25ms. The actual turn-on time will vary slightly with input voltage, but the power-up sequence is independent of the load at either output. Using the On/Off Enable Controls on the PT4660 and PT4680 Series of DC/DC Converters The PT4660 (48V input) and PT4680 (24V input) series of 75-W dual-output DC/DC converters incorporates both positive and negative logic Output Enable controls. EN1 (pin 3) is the positive enable input, and EN2 (pin 4) is the negative enable input. Both inputs are TTL logic compatible, and are electrically referenced to -Vin (pin 2) on the primary (input) side of the converter. A pull-up resistor is not required, but may be added if desired. Adding a pull-up resistor from either input, up to +Vin, will not damage the converter. Automatic (UVLO) Power-Up Connecting EN1 (pin 3) to -Vin (pin 2) and leaving EN2 (pin 4) open-circuit configures the converter for auto- matic power up. (See data sheet “Typical Application”). The converter control circuitry incorporates an “Under Voltage Lockout” (UVLO) function, which disables the converter until the minimum specified input voltage is present at ±Vin. (See data sheet Specifications). The UVLO circuitry ensures a clean transition during power-up and power-down, allowing the converter to tolerate a slow- rising input voltage. For most applications EN1 and EN2, can be configured for automatic power-up. Positive Output Enable (Negative Inhibit) To configure the converter for a positive enable function, connect EN1 (pin 3) to -Vin (pin 2), and apply the system On/Off control signal to EN2 (pin 4). In this configura- tion, a logic ‘0’ (-Vin potential) applied to pin 4 disables the converter outputs. An example of this configuration is detailed in Figure 1. Negative Output Enable (Positive Inhibit) To configure the converter for a negative enable function, EN2 (pin 4) is left open circuit, and the system On/Off control signal is applied to EN1 (pin 3). A logic ‘0’ (-Vin potential) must then be applied to pin 3 in order to PT4660 EN 1* EN 2 –Vin –V IN 1 =Outputs Off 4 3 2 BSS138 PT4660 EN 1* EN 2 –Vin – V IN 4 3 2 1 =Outputs On BSS138 Figure 2; Negative Enable Configuration Figure 1; Positive Enable Configuration During turn-off, both outputs drop rapidly due to the discharging effect of actively switched rectifiers. The voltage at Vo1 remains higher than Vo2 during this pe- riod. The discharge time is typically 100µs, but will vary with the amount of external load capacitance. enable the outputs of the converter. An example of this configuration is detailed in Figure 2. Note: The converter will only produce and output voltage if a valid input voltage is applied to ±Vin. |
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